With the development in digital technologies in recent years, electronic devices, such as mobile information equipment and information home appliances have higher functionality. Thus, demands for a non-volatile storage element which has a greater capacity, higher writing and reading speed, and longer-life and which consumes lower power in writing have been increased.
To meet such demands, efforts in miniaturizing flash memories using existing floating gates are said to have limitations. Accordingly, attention is recently focused on a new variable resistance nonvolatile memory element including a variable resistance layer as a material of a storage unit.
The variable resistance nonvolatile memory element has a very simple structure including a variable resistance layer that is disposed between and in contact with a lower electrode and an upper electrode. A resistance state changes between a low resistance state and a high resistance state only with application, between the lower electrode and the upper electrode, of a predetermined electric pulse having a voltage equal to or higher than a threshold. Then, information is recorded in association with these different resistance states and values. Since the variable resistance nonvolatile memory element has such a simple structure and simply performs operations, it is expected that the nonvolatile memory element can further be miniaturized and the cost can be reduced. Since the resistance state sometimes changes between the low resistance state and the high resistance state in order of length of time not longer than 100 nano-seconds, the attention is further focused on the variable resistance nonvolatile memory elements in view of its higher operating speed, and various proposals have been made.
In particular in recent years, there are many proposals of variable resistance nonvolatile memory elements using metal oxides in variable resistance layers. The variable resistance nonvolatile memory elements using metal oxides can be largely divided into two types, depending on a material to be used in each variable resistance layer. One is the variable resistance nonvolatile memory elements using perovskite materials (Pr(1-x)CaxMnO3 [PCMO], LaSrMnO3 [LSMO], and GdBaCoxOy [GBCO], for example) in the variable resistance layers, as disclosed in PTL 1 and others.
The other is the variable resistance nonvolatile memory elements that are compounds comprising an only transition metal and oxygen, using binary transition metal oxides. Compared to the perovskite materials, the binary transition metal oxides have very simple composition structures. Thus, controlling the compositions when manufactured and forming the films are relatively easy. In addition, with the advantage of relatively favorable compatibility with semiconductor manufacturing processes, the variable resistance nonvolatile memory elements have intensely been studied in recent years.
For example, PTL 2 discloses a variable resistance nonvolatile memory element using, as variable resistance materials, (i) transition metal oxides of stoichiometric composition, such as nickel (Ni), niobium (Nb), titanium (Ti), zirconium (Zr), hafnium (Hf), cobalt (Co), iron (Fe), copper (Cu), and chrome (Cr), and (ii) oxides whose composition is deficient in oxygen compared to its stoichiometric composition (hereinafter referred to as oxygen-deficient oxides). Furthermore, PTL 3 discloses the variable resistance nonvolatile memory element using an oxygen-deficient tantalum (Ta) oxide as a variable resistance material. When a Ta oxide layer is denoted as TaOx, PTL 3 reports a resistance change phenomenon in a range satisfying 0.8≦x≦1.9 (from 44.4% to 65.5% in terms of oxygen concentration).
Here, the oxygen-deficient oxides will be described in detail. For example, in the case of Ta, Ta2O5 is known as an oxide having a stoichiometric composition. Ta2O5 includes Ta atoms and O atoms in a 2:5 ratio, and the oxygen content percentage of Ta2O5 is 71.4 atm. %. An oxide with an oxygen content percentage lower than 71.4 atm. % is called an oxygen-deficient tantalum oxide. In this example, since Ta2O5 is an oxide of Ta, it can be represented as an oxygen-deficient Ta oxide.